1. Field of the Invention
This invention relates to a muting circuit, and more particularly to a muting circuit effective to use for an amplifier.
2. Description of the Prior Art
In the prior art, it has been known that pop noise is generated at the output terminal of an amplifier in response to the rising-up and falling-down transition characteristics of a voltage supplied from a voltage source to the amplifier, and the pop noise is reproduced from a speaker to give unpleasant feeling to a listener with the result that the speaker may be damaged. For this reason, the output signal of the amplifier is usually muted for a predetermined time after the ON-OFF manipulation of a voltage source. A muting circuit of this kind is disclosed in U.S. Pat. No. 3,828,267 patented on Aug. 6, 1974 which has the same inventor and assignee as the present application. In the muting circuit disclosed in this patent, a switching transistor is provided between the output terminal of the amplifier and a reference point, and the transistor is turned on for a predetermined time after the ON-OFF manipulation of the voltage source so that a signal at the output terminal is not supplied to the speaker for a predetermined time. In this case, one part of the output signal leaks through the base-collector junction of the transistor at its nonconductive time during negative half cycles of the output signal of the amplifier, so that the output signal could be distorted. For this reason, in the prior art, the base electrode of the transistor is applied with a cut-off bias voltage larger than the peak value during the negative cycle. However, when the aforesaid output amplifier is a large output amplifier, the peak value of the output signal becomes large thereby increasing the cut-off bias voltage supplied to the base electrode, so that the collector-opened base-emitter withstanding voltage BV.sub.EBO of the switching transistor is required to be larger than the value of BV.sub.EBO of the switching transistor cannot be derived from the amplifier and hence a switching transistor used in the muting circuit of a large output amplifier is desired to have a sufficiently large BV.sub.EBO.